Present turbomachines are provided with an annular combustion chamber having as its axis of symmetry the main axis of the turbomachine. One such chamber is shown in FIG. 5. The combustion chamber is typically defined by an end wall 12 including fuel injectors 13 and oxidizing air inlets, and by an annular wall 15 that extends in the longitudinal direction of the chamber 10 (thus corresponding to the upstream to downstream direction), substantially parallel to the main axis A of the turbomachine (not shown). The chamber 10 is closed at its upstream end by the end wall 12, and it is open at its downstream end 17 in the longitudinal direction to enable the burnt gases to be exhausted. This annular wall 15 is typically constituted by an annular inner shroud (a radially inner wall) 151 and by an annular outer shroud (radially outer wall) 152. The inner shroud 151 and the outer shroud 152 are coaxial about the main axis A of the turbomachine, the inner shroud 151 being closer to the main axis of the turbomachine than is the outer shroud 152, i.e. having a radius that is smaller than the radius of the outer shroud 152.
Upstream from the end wall 12, an upstream annular inner wall 11 of the chamber 10 extends the inner shroud 151 upstream.
The annular wall 15 is pierced over its entire area (or over a major fraction thereof) by a plurality of orifices of greater or smaller size, which orifices are to allow air to penetrate into the combustion chamber 10. The air that flows along the inner shroud 151 on the outside of the chamber 10, and that subsequently penetrates into said chamber via these orifices, flows between said inner shroud 151 and a wall referred to as the inner flange 21 of the chamber. This inner flange 21 that is annular and coaxial with the inner shroud 151 of the chamber, thus has a radius that is smaller than the radius of the inner shroud 151. The inner flange 21 is pierced by orifices, some of which (upstream orifices 215) are situated in its upstream portion, substantially facing the central portion of the inner shroud 151 of the chamber 10 (i.e. half way between the end wall 12 of the chamber 10 and the downstream end 217 of the inner flange 21). Thus, the air flowing along the inner shroud 151 passes in part via these upstream orifices 215. Once it has passed through these upstream orifices, this air cools the high-pressure (HP) turbine wheel that is situated downstream therefrom.
Because of this disposition of the inner wall of the combustion chamber and because of the orifices in the inner flange, the flow of air for passing through the orifices in the inner flange in order to cool the HP turbine wheel is subjected to the influence of the combustion chamber. Before passing through these orifices, this air is in contact with the inner wall, which is hot and which is also pierced by air inlet orifices, and this air is thus subjected to heating by convection. This air is also subjected to heating by radiation through these orifices in the chamber, the radiation coming from the flames of the combustion. In addition, the instabilities of the combustion generate turbulence in the flow of air, through the orifices in the chamber, which turbulence can contribute to disturbing the feed of cooling air to the HP wheel.
Overall, this air is thus subjected to heating that is harmful since the function of the air is to cool the HP turbine wheel.